1. Field of the Invention
The present invention relates to a vehicle steering system, and more particularly to a vehicle four wheel steering system. More specifically, the present invention pertains to a vehicle four wheel steering system in which the steering angle ratio between the front and rear wheels is changed in accordance with the vehicle operating conditions such as the vehicle speed.
2. Description of the Prior Art
In the field of motor vehicles, the concept of four wheel steering is old. It has also been known to steer the rear wheels in different ways with respect to the front wheels depending on the vehicle operating condition such as the vehicle speed. For example, the U.S. Pat. No. 2,910,131 discloses a vehicle four wheel steering system in which the rear wheels are steered together with the front wheels. The U.S. patent teaches to steer the rear wheels in the direction opposite to the direction in which the front wheels are steered when the vehicle speed is low but to steer the rear wheels in the same direction as the direction in which the front wheels are steered when the vehicle speed is high.
Japanese patent application 53-163678 which has been filed on Dec. 29, 1978 and disclosed on July 11, 1980 under the disclosure number 55-91457 discloses a vehicle four wheel steering system in which the steering angle ratio, that is, the ratio of the steered angle of the rear wheels to the steered angle if the front wheels is changed in accordance with the vehicle speed so that the rear wheels are steered in the direction opposite to the direction of steering of the front wheels under a low vehicle, speed range and in the in the same direction as the direction of steering of the front wheels. This Japanese patent application corresponds to the U.S. Pat. No. 4,313,514 issued to Y. Furukawa et al on Feb. 2, 1982.
It should be noted that the known four wheel steering systems do not take into account the road conditions on which the vehicle is running. In the prior art steering systems described above, therefore, desirable steering characteristics cannot be obtained under slippery road conditions if the system is designed to provide an optimum result in a dry road condition. In view of the above problems, the Japanese patent application 59-4597 filed on Jan. 13, 1984 and disclosed for public inspection on Aug. 6, 1985 under the disclosure number 60-148769 proposes to control the four wheel steering system in accordance with the road condition such as the coefficient of friction between the vehicle wheels and the road on which the vehicle is running.
According to the proposal by the Japanese patent application, the coefficient of friction is detected as an example in terms of the lateral acceleration produced in the vehicle body during a cornering of the vehicle and the steered angle of the vehicle. There is further taught that under a low frictional coefficient such as in an icy road and where the vehicle is to be steered to conduct a lane change on a straight path, the rear wheel steering signal is modified by being added with a modification signal which produces in the rear wheels a steering movement in the direction of steering of the front wheels. This will mean that the rear wheel steering angle will be increased where the rear wheels are steered in the same direction as the direction of steering of the front wheels whereas the rear wheel steering angle will be decreased where the rear wheel steering direction is opposite to the direction of steering of the front wheels. The teachings in the Japanese patent application are such that the strength of the modification signal is determined so that the fish tailing of the vehicle during the lane change can be avoided.
The Japanese patent application further teaches to determine the modification signal so that the rear wheels are given with a steering angle in the direction opposite to the direction of steering of the front wheels in case where the frictional coefficient is low and the vehicle is to be passed through a sharp turn. The amount of the modification signal is determined so that the front wheels do not lose the road gripping power.
It should however be noted that the teachings in the Japanese patent are not precise and does not describe how the modification signal is determined under various road conditions. It should further be pointed out that the Japanese patent application teaches to provide manual means for determining the direction of the modified steering movement of the rear wheels. In other words, according to the teachings by the Japanese patent application, the driver has to judge as to whether the vehicle is in the lane change condition wherein the rear wheel steering shall be modified by being added with an additional steering angle in the direction of steering of the front wheel or in the sharp cornering condition wherein the rear wheel steering shall be modified by being added with an additional steering angle in the opposite direction of the steering of the front wheel. Further, the driver has to operate the manual means based on his judgement so that a desired modification is made on the rear wheel steering.
Another problem in the known four wheel steering system is that in a case where the steering ratio between the front wheel steering angle and the rear wheel steering angle is controlled solely in accordance with the vehicle speed, the rear wheel steering direction is changed from the direction opposite to the direction of the steering of the front wheels to the direction which is the same as the steering direction of the front wheels as the vehicle speed is increased in a curved path. This tendency of the vehicle steering characteristics will give the driver an increasingly strong feeling of under steering.